The present invention relates to manufacture of a .beta.-BaB.sub.2 O.sub.4 (beta barium borate) thin film utilizing sol-gel preparation.
In recent years, laser technology has been an essential factor in various fields including medicine, processing, and nuclear fusion, and development of laser for potent ultraviolet rays highly required. Laser with enhanced function of the wavelength tunability is necessary for efficient photochemical reactions, and optical information processing, and optical instrumentation, and optoelectronics demand improved, small and durable laser equipment.
Under such circumstances, solid state lasers combined with non-linear optical material for wavelength conversion have been proposed to generate ultraviolet rays in place of bulky liquid lasers or gas discharge lasers, with relatively shorter lives. Various inorganic and organic non-linear optical materials have been tested for the efficiency of wavelength conversion, and beta-barium borate (.beta.-BaB.sub.2 O.sub.4, hereinafter referred to as BBO) with second harmonic generation (SHG) characteristics has been found to be especially effective.
BBO is an SHG element which emits ultraviolet rays at wavelengths shorter than 200 nm through harmonic generation, and has two crystal structures, that is, an alpha phase and a beta phase. While the alpha phase is a stable crystal phase generated at relatively high temperatures, the beta phase is a metastable crystal phase generated at relatively low temperatures and has non-linear optical activities.
A traditional technique for preparing the beta phase of BBO is known as the flux method, in which a flux element is added to BBO molten salt to decrease the melting point and to yield the BBO beta phase crystal from the molten salt.
Another known technique is the "Czochralski method", in which beta-phase, single-crystal BBO is directly grown and pulled by selecting optimal starting materials and conditions for molten salt holding and crystal growth.
These conventional methods, however, demand delicate and precise condition controls based on the melt structure of the BBO composition to obtain high quality crystals from the molten salt.
The metastable beta crystals are obtained from the molten salt only by selecting with great care starting materials with excellent beta crystalline characteristics and optimal growth conditions. The crystal obtained often includes impurities and may have heterogeneous crystal structure or thermal distortion.
These known methods can be applied to preparation of single BBO crystals, but are not capable of BBO thin film which is useful for integrated circuits.